by both means combined; and it has been further accompanied by greater attention to the important matter of providing carefully against losses by conduction and radiation, and by internal wasteful transfer of heat. The use, finally, of the "compound" or double-cylinder engine for the purpose of reducing friction, as well as of saving some of that heat which is usually lost in consequence of internal condensation and reëvaporation due to great expansion, has already been considered when treating of the marine engine.
It is evident that, although there is a limit, which is tolerably well defined, in the scale of temperature, below which we cannot expect to pass, using the now standard type of engine, a degree gained in approaching this lower limit is more remunerative than a degree gained in the range of available temperature, by increasing the maximum temperature. Hence, the attempt made by the French inventor, Du Trembly, a quarter of a century ago, and by other inventors since, to utilize a larger proportion of heat by approaching more closely the lower limit, was in accordance with what are now well-known scientific principles.
The form of engine here referred to is known among engineers as the Binary Vapor-Engine. In it the heat usually carried away by the water delivered from the condenser of the steam-engine is made to evaporate some very volatile liquid, as ether or carbon bisulphide, which, in turn, by the expansion of its vapor, develops additional mechanical power. Mechanical difficulties have hitherto prevented the success of this form of engine; but it cannot be pronounced impossible that coming inventors may make the system commercially valuable.
An important consequence of the still unchecked rise of piston-speed in the modern steam-engine is the approach to a limit beyond which the now standard form of "drop cut-off," or "detachable" valve-gear, cannot be used. For the piston would, at that limit of speed, reach the end of its stroke before the dropped valve could reach its seat, and the point of cut-off and degree of expansion could no longer be determined accurately and invariably by the governor. This limit has probably already been attained in some engines; and the engineer adopting such piston-speeds as 1,000 feet per minute or more is driven back to the use of the older types of "positive-motion" valve-gearing, and is compelled to devise special forms of governor which shall have sensitiveness, and yet power sufficient to control these less tractable kinds of mechanism, and to invent reliable and durable forms of balanced valves, and to practise every practicable expedient for making the movement of the valve, and its adjustment by the regulator, perfectly easy. Positive motion and ease of adjustment by the governor are, therefore, evidently the requisites of a successful valve-gear for the engine which will probably succeed the standard engine of to-day.
We may now summarize the results of our examination of the growth of the steam-engine thus:
1. The process of improvement has been one, primarily, of "dif-
